The present invention relates generally to semiconductor manufacturing, and more particularly to an illumination system that is locally spatially controllable to reduce line width variations in a photolithographic system.
Photolithography is often used in the manufacture of semiconductors. The image of a reticle having a circuit pattern thereon is projected onto a photosensitive substrate such as a wafer. The wafer is subsequently processed, forming printed patterns thereon. This process is generally repeated several times, forming multiple layers. As the art develops and the element sizes of the semiconductor devices become smaller, there is a need for improved illumination sources used in the lithographic printing systems. Accordingly, there have been many types of illumination systems disclosed which may enhance the quality of imaging of a reticle onto a photosensitive substrate. One such illumination system is disclosed in U.S. Pat. No. 5,296,892 entitled xe2x80x9cIlluminating Apparatus and Projection Exposure Apparatus Provided With Such Illumination Apparatusxe2x80x9d issuing to Mori on Mar. 22, 1994, which is herein incorporated by reference. Therein disclosed is an illumination system having an optical integrator or fly""s eye lens positioned next to a condenser. The optical integrator or fly""s eye lens is designed to be replaceable so that the numerical aperture of the illumination system can be varied. Another illumination system is disclosed in U.S. Pat. No. 5,357,312 entitled xe2x80x9cIlluminating System In Exposure Apparatus For Photolithographyxe2x80x9d issuing to Tounai on Oct. 18, 1994, which is herein incorporated by reference. Therein disclosed is an illumination system that desirably shapes a uniform light beam rather than by blockage with an aperture diaphragm thus reducing loss of illuminating light. Additionally, the cross sectional intensity profile of the light beam in the source aperture can be rendered annular. Yet another illumination system is disclosed in U.S. Pat. No. 5,329,336 entitled xe2x80x9cExposure Method and Apparatusxe2x80x9d issuing to Hirano et al on Jul. 12, 1994. Therein disclosed is an exposure apparatus having a detector that is compensated for due to coherence changes in the exposure apparatus. Additionally, the concept of zone and modified light source illumination is disclosed for the purpose of optimizing the image of a reticle pattern. Accordingly, there is a need to provide different illumination characteristics to reduce variations of line width due to the photolithographic system or tool. However, due to the complexity of illumination sources and the difficulty of modifying them for a particular optical system to compensate for variations of the photolithographic system, and in particular the projection optics, often the image is not optimized. As a result, some portions of the field may be illuminated adequately while others are not. This may result in different imaging results on different portions of a wafer. Additionally, the projection optics may not provide an image of a reticle onto a photosensitive substrate that when processed has line widths that are within a predetermined tolerance. Therefore, there is a need for an illumination system that compensates for the projection optics variation to optimize the exposure of a photosensitive substrate.
The present invention relates to the use of an optical element which may be a microlens array or diffractive element that is divided into a plurality of different spatial regions, providing different illumination characteristics or properties for each local region. The optical element with its plurality of different regions is matched to different regions of line width variance due to different optical performance at the local area in question that result from variances in the photolithographic system or tool, and in particular the projection optics. The location of the different illumination characteristics are selected to optimize or provide improved imaging at different locations in the image field. Each of the plurality of different illumination regions on the optical element is matched to each of the plurality of different locations in the image field to locally modify the illumination of a reticle resulting in an overall improvement of the image of the reticle on a processed photosensitive substrate. The present invention is particularly applicable to a scanning photolithographic system.
It is an object of the present invention to optimize the performance of a photolithographic system, and in particular to reduce line width variations.
It is another object of the present invention to compensate for certain imperfections in the optics of a photolithographic tool that affect line width.
It is a further object of the present invention to be able to adjust the image line width selectively in the horizontal or vertical axis at different locations in a printed field.
It is an advantage of the present invention that the illumination system can easily be changed to improve imaging or adapt to changes in the projection optics.
It is a feature of the present invention that different illumination regions of an optical element are used to locally modify illumination of a reticle and improve performance of the photolithographic tool or system.
It is another feature of the present invention that the different illumination regions modify the emerging cone of radiation or illumination used to project the image of a reticle onto a photosensitive substrate.
It is another feature of the present invention that partial coherence or fill geometry is locally or spatially modified in an illumination field.
These and other objects, advantages, and features will become readily apparent in view of the following detailed description.